Closed-Loop Crop Cascade to Optimize Nutrient Flows and Grow Low-Impact Vegetables in Cities

Abstract

Urban agriculture systems can significantly contribute towards mitigating the impacts of inefficient and complex food supply chains and increase urban food sovereignty. Moreover, improving these urban agriculture systems in terms of nutrient management can lead to a better environmental performance. Based on a rooftop greenhouse in the Barcelona region, we propose a cascade system where the leachates of a tomato cycle from January to July (donor crop) are used as the main irrigation source for five successive lettuce cycles (receiving crop). By determining the agronomic performance and the nutrient metabolism of the system, we aimed to define the potential of these systems to avoid nutrient depletion and mitigate eutrophication, while scaling the system in terms of nutrient supply between the donor and the receiving crops. The results showed that low yields (below 130 g per lettuce plant) are obtained if a cascade system is used during the early stage of the donor crop, as the amount of nutrients in donor’s leachates, specially N (62.4 mg irrigated per plant in the first cycle), was not enough to feed the lettuce receiving crop. This effect was also observed in the nutrient content of the lettuce, which increased with every test until equaling the control (4.4% of N content) as the leachates got richer, although too high electrical conductivity values (near 3 dS/m) were reached at the end of the donor crop cycle. Findings on the uptake of the residual nutrient flows showed how the cascade system was able to take advantage of the nutrients to produce local lettuce while mitigating the effect of N and P in the freshwater and marine environments. Considering our case study, we finally quantified the scale between the donor and receiving crops and proposed three major ideas to optimize the nutrient flows while maintaining the yield and quality of the vegetables produced in the receiving crop.

Improving the Fertigation of Soilless Urban Vertical Agriculture Through the Combination of Struvite and Rhizobia Inoculation in Phaseolus vulgaris

“…Moreover, 25% of the total sampled leaves for each plant were separated to determine their areas before the drying process. To do so, these fresh leaves were scanned with a reference pixel to obtain leaf areas using ImageJ software (Rueden et al, 2017). These leaf areas were further extrapolated to 100% of the leaf biomass of the plant.…”

“…The widespread use of these nutrients has caused vertical farming to rely entirely on them, which thus makes this agricultural practice unsustainable in the long run. The high energy cost of synthetic nitrogen production and the ever-depleting sources of phosphate rock, when added to the environmental cost of their disposal and emissions to water and air ( Rufí-Salís et al, 2020a , b ), necessitates the search for alternatives to further implement these technologies in a sustainable way.…”

Water Nutrient Management in Soilless Plant Cultivation versus Sustainability

“…Unfortunately, their application faces serious challenges, like changes in the composition of the nutrient medium that has passed through the system, as well as the presence of pathogens and root secretions, including phytohormones, that inhibit plant development [6]. There are two main ways of managing the overflow in closed circuits: recirculation-using DW in the cultivation of the same crop (Figure 3), and the so-called cascade systems-using DW in the cultivation of another, less demanding crop (Figure 4) [42].…”

“…In this case, the recirculation system requires constructing stations for, i.e., disinfection or desalination of the nutrient medium or water used for dilution. Otherwise, DW recirculation can diminish the quantity and quality of crop yields that greenhouse owners cannot afford [42].…”

“…In this case, the system enabled saving water and removing nitrates, but caused a decrease in rosemary yield compared to the cultivation variant with a standard nutrient medium. A key parameter for planning the number and type of plants that can be used in secondary cultivation is the variability of nutrient contents in the outflow from the primary cultivation [42]. A safe solution for cascade systems is to use DW from the soilless cultivation of field crops.…”